If you have a serial number you can right click Computer then scroll down and you will see Windows is not activated. Click that link and a window will open allowing you to enter your serial number. Make sure that you are connected to the Internet while performing this operation.
If you want to use the OS in its trial mode for 120 days, you can use slmgr -rearm command in a Command Prompt window with Administrator access. You can activate Windows 7 for 120 days using 3 re-arms.